A trail of cosmic dust may lead to alien life

A vast, salty ocean lies hidden deep within a frozen moon of Saturn

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Outer space may look like an empty void. But that is not completely true.

The vast space between the planets, moons and stars hold trillions of specks of dust. These grains are few and far between. And they are too small to see with the naked eye. But they are fast. They shoot through space as quickly as 400 kilometers (250 miles) per second. That’s hundreds of times faster than a bullet. Importantly, they hold lots of information about places far, far away.

That's why Sascha Kempf grew excited when a NASA space probe named Cassini ran into clouds of dust as it sped toward Saturn in 2004. Kempf is a planetary scientist at the University of Colorado in Boulder. He worked on the Cassini mission.

In this image, made by the Cassini spacecraft in 2007, the moon Enceladus appears to hover above Saturn’s rings.

NASA/JPL/Space Science Institute

Cassini blasted off from Florida in 1997. By February 2004, it had traveled through more than two billion kilometers (1.2 billion miles) of space. At that point, it began to radio back clues of something strange. Cassini detected the chemical signature of a mineral common on Earth — silicate — in the distant space dust. Silicate is the stuff from which beach sand is made.

“That’s really weird,” thought Kempf, as he looked at the data arriving back home. He knew Saturn and everything around it was rich in water. "All the moons we are aware of are ice moons. Saturn's rings are ice rings," he points out. And yet Cassini was picking up little pieces of rock. He felt both excited and worried.

Other scientists asked if his discovery was too strange to be true. Maybe Cassini's dust detector was malfunctioning. Maybe it had gotten contaminated with Earth dust before it blasted off. "I never really felt comfortable," Kempf says, "because it didn't make sense."

Years later, he is finally breathing a sigh of relief. Those whiffs of mineral dust no longer look like a grand scientific mistake. Instead, they have revealed a major discovery. Locked deep inside one of Saturn's frigid moons, Enceladus, is a warm place where alien life might thrive.

Old trash

Scientists have known for more than 50 years that outer space is filled with flying dust. Apollo 11 astronauts brought back rocks from the moon’s surface in 1969. Those rocks were dotted with tiny pits and craters. Each of those holes had formed when a grain of cosmic dust smacked into the surface of the moon. They hit so hard they melted a bit of the rock into glass.

Rocks brought back from the moon are covered in tiny craters. These craters formed as tiny pieces of space dust crashed into the moon

NASA History Program

Some of this stray dust is likely older than Earth. It is leftover debris from the stuff that clumped together more than four billion years ago to form Earth and the other planets. Other dust comes from newer sources. Meteors kicked it into space when they smacked into planets or moons.

Engineers designed Cassini so it could survive running into odd bits of fast-moving dust. They also fitted it with equipment to count the dust grains and analyze what they were made of.

As Cassini approached Saturn in early 2004, the spacecraft was only capturing one or two bits of dust per week. Most people would consider this boring. But not Kempf’s team. They specialize in studying cosmic dust. And they celebrated every time Cassini caught another mote of it. “Our colleagues made fun of us,” Kempf admits.

Until, that is, things got interesting.

In mid-March 2004, Cassini passed through a cloud of dust so thick that it detected a grain every couple of minutes. The dust detector then went quiet after several days. Two weeks later, another patch of dust clouded its path. More clouds followed.

It was unusual dust. Its recipe included silicate minerals like those found on Earth.

The clouds of mineral dust grew thicker as Cassini approached Saturn. Kempf and his team were eager to find out where this dust could be coming from. Saturn offered plenty of possibilities.

A snow-covered moon

As planets go, Saturn is a veritable cosmic pinball machine. At least 50 little moons whiz around it. Some even careen through the narrow gaps between the planet's rings. Once Cassini arrived at Saturn in late 2004, flight controllers at NASA began to steer it past one moon after another to get some closer looks.

Enceladus, a moon of Saturn, is constantly coated in new sprinkles of snow. This snow reflects sunlight, making Enceladus the brightest object in the solar system.

NASA/JPL-Caltech

A flyby on July 14, 2005, took Cassini past Saturn's sixth-largest moon, Enceladus (En-SEL-uh-dus). This little marble of a world is barely 500 kilometers (310 miles) across. That makes it small enough to fit within the borders of Texas. It’s a moon that had already captured scientists’ interest.

Early on, they "realized that something is odd about Enceladus," notes Kempf. Parts of its surface were covered in craters — as if nothing had changed for billions of years. Other regions had almost no craters. At those sites, something had reshaped the surface and erased old craters.

Enceladus also was known to be the brightest, whitest object in the solar system. Imagine the glare that you see on a snow-covered meadow on a bright afternoon. Cover your face, and you still see it burnt into your eyes. The glare off of Enceladus comes from the reflection of sunlight off of its snow.

This is weird because the moon has almost no atmosphere. It has, in fact, no real weather. It certainly lacks the clouds from which snow falls on Earth. But somehow, a steady sprinkle of snow is always raining down on this moon.

Tiger stripes

Traveling 29,500 kilometers (18,300 miles) per hour, Cassini streaked by within 168 kilometers (104 miles) of the moon's surface. It captured the first-ever close-up photos of the moon’s south pole. And those pictures turned up a surprise.

Four deep cracks cut across the south pole of Enceladus. Jets of ice crystals and water vapor constantly squirt from those cracks.

NASA/JPL-Caltech

Cassini's cameras revealed a rugged landscape unlike the rest of Enceladus. The area around its south pole was squished, faulted and buckled into ridges and valleys. Ice boulders larger than houses were strewn about. And four massive cracks sliced across the area. Each crack was straight and deep, like the cuts from a tiger's claw — and 130 kilometers (around 80 miles) long. A thin mist of ice crystals and water vapor sprayed out from these cracks. They resembled geysers on Earth.

Cassini's dust detector scooped up thousands of those ice crystals. They contained water ice, like the snow falling on Earth. Many also held sodium chloride — the main ingredient of sea salt.

This confirmed what scientists had speculated. The surface of Enceladus is –200° Celsius (about –330° Fahrenheit). That's almost cold enough to cause air on Earth to turn liquid. But the moon was warm enough on its inside to hold a vast, salty ocean buried under 40 kilometers (25 miles) of ice.

"We don't really understand how that ocean has survived," says Francis Nimmo. "We would have expected [it] to have frozen after a few million years." Instead, that ocean has probably been around since the moon first formed, four billion years ago.

Cassini has sampled the ice crystals and vapor that spew like geysers out of cracks in the surface of Enceladus. These materials provide evidence of a salty ocean hidden deep within the moon.

NASA/JPL-Caltech/Space Science Institute

Nimmo is a planetary scientist at the University of California, Santa Cruz. He studies the insides of moons. Enceladus gets squeezed and stretched by gravity as it orbits Saturn, he suspects. "Some of that squeezing and stretching gets converted into heat," he says. Such heat might keep the ocean from freezing.

Most of the ice spraying out of Enceladus falls back onto it. This explains its constant coating in fresh, white snow. Some of the snow also settles onto four nearby moons: Mimas, Tethys, Dione and Rhea. This gives them a faint, reflective sheen.

Some ice crystals from Enceladus even become part of Saturn's icy outer, E-ring. The moon's icy eruptions likely created this, the largest of the planet’s rings. It forms a circle two million kilometers around.

Once ice crystals join the E-ring, radiation emitted by Saturn gradually eat away the crystals. Each bit of ice likely persists in the ring for 100 to 200 years. That implies the E-ring survives only because Enceladus constantly belches more ice into it. All told, this little moon may feed the E-ring 45 million kilograms (50,000 tons) of ice per year. Melt that ice into water, and it would fill 18 Olympic-sized swimming pools.

Journey of dust

Kempf looked at the makeup of ice crystals that Cassini sampled from Enceladus and the E-ring. Ice from both places had bits of silicate dust within it. This was the same stuff Cassini had encountered 50 million kilometers (31 million miles) out from Saturn.

Those silicate grains must come from the ocean deep inside Enceladus, Kempf realized. The moon expels them wrapped inside crystals of ice. Once they enter the E-ring, the ice is cooked away by Saturn’s radiation. This leaves behind only the dust. Being bombarded by radiation, those dust grains develop an electric charge. Saturn's strong electric field eventually hurls this dust far into space — where Cassini had first run into it.

Ice crystals from Enceladus end up in Saturn’s outer ring, the E-ring. The planet’s electric field then flings them out into space.

NASA

It might have been tempting to forget about the dust at that point and move on to something else. But planetary scientists like Kempf are different from many others. A geologist studying something on Earth can usually go out and collect more rocks without much trouble. But studying a faraway moon, Kempf can't do that. He must make do with what few data are available. So Kempf wondered, what might these tiny silicate grains tell him about the interior of Enceladus?

The dust specks that Cassini had tasted were long gone. They had vaporized the instant they smacked into the spacecraft’s detector. But the researchers had information about the dust’s chemical recipe. Over several years, Kempf and two colleagues looked at data from more than 10,000 of those specks.

They gradually realized that the dust grains from Enceladus were unusual in several major ways.

Unlike most rocks on Earth, they contained almost no traces of common mineral-forming metals such as calcium, magnesium and iron. They also were all about the same size — roughly one thousandth as wide as a red blood cell. The dust flung into space when meteors hit moons or planets usually spans a wide range of sizes. So the uniform size of those leaving Enceladus “indicated a very peculiar origin of those particles,” notes Sean Hsu.

At the time, Hsu was working on his PhD under Kempf at the University of Colorado in Boulder (where he still works today). He spent months poring over old studies to see how these tiny, pure-silicate particles form on Earth. High temperature proved key. They grow in hydrothermal vents on the seafloor. These vents spew hot water from deep inside the Earth. Kempf and Hsu reported this in March 2015.

Scientists have discovered unusual hot water vents at the bottom of the Atlantic Ocean, such as this stone chimney rising three stories above the sea floor. Similar vents may sit at the bottom of Enceladus’s ocean, providing a habitat for life.

D.S. Kelley and M. Elend, University of Washington

Like Earth, Enceladus must have hydrothermal vents bubbling on its seafloor, they concluded.

A team of scientists in Japan followed up with some convincing laboratory experiments. Working with sealed bags containing water and minerals, they ran mini versions of the chemical reactions that happen in hydrothermal vents. These showed that deep inside Enceladus, where the silicate grains are made, the water must be at least 50 °C (122 °F), as hot as a scalding hot tub. But it might even be boiling hot — 100 °C.

"No one would have expected that this is the explanation back in 2005," says Kempf. He had assumed that the interior of Enceladus was barely warm enough to keep its ocean unfrozen. And many people had believed it was only a small ocean, limited to the moon’s south pole area. But in September 2015, NASA released Cassini data showing evidence of a much larger, global ocean of waterbeneath the frozen crust of this moon.

Nor is this necessarily the only Saturnian moon to host a subsurface sea. On October 7, Cassini data provided that Dione, too, might hide an ocean beneath its icy crust. In fact, at least half a dozen objects in our outer solar system are now believed to hold oceans beneath frozen shells. This includes Jupiter’s moons Europa and Ganymede, Neptune’s moon Triton, and even the dwarf planet Pluto.

"Life is full of surprises," says Kempf.

Lost Atlantis

If there really are warm places inside Enceladus, then it raises the chance that this moon might host life. After all, living things don’t just need water; they also need food — energy. And hydrothermal vents can supply lots of food. Deborah Kelley saw this first-hand in 2000. She’s an oceanographer at the University of Washington in Seattle. And during an expedition in the Atlantic Ocean, her team discovered a new type of seafloor vent.

The Lost City Hydrothermal Field is named after the mythical sunken city of Atlantis. It sits atop an undersea mountain named the Atlantis Massif. Its cluster of vents looks like the skyline of a medieval city. Hot water there bubbles out of chimneys. Their pointed stone spires resemble those of a castle. Some tower up to 60 meters (200 feet) above the seafloor.

Thick, fuzzy blobs of microbes decorate the chimneys. Living in the nooks and crannies of their walls are worms, snails, clams and little shrimpy amphipods. Prowling around for food are eels, wreckfish with spiny fins and crabs with spidery legs.

The lost city hydrothermal vents support a rich array of life, including this geryonid crab.

D.S. Kelley, University of Washington, IFE, URI-IAO, and NOAA

Kelley found that two gases, hydrogen and methane, trickle from the vents. Both make excellent food for microbes. This is why these vents are such hotspots for life. The chemical reactions that produce these gases deep inside Earth are known by the termserpentinization (SUR-pen-teen-ih-ZAY-shun). Scientists believe that the same reaction occurs below the seafloor of Enceladus.

These gases could easily feed living things inside that ice-covered moon, says John Priscu. He is a microbiologist at Montana State University in Bozeman. He has studied single-celled microbes in some of the harshest places on Earth. Those sites include Antarctica's ice.

"These oceans in the outer solar system should have life," he says. "If it wasn't there, I would be very surprised." Such life may amount to no more than microbes. After all, they require very little energy. "But if there are geothermal vents, there could even be multicellular organisms," Priscu says. By that he means bigger organisms, like the worms and shellfish seen at Lost City.

Looking for life

Last fall, on October 28, Cassini pulled off one of its most impressive acrobatic stunts. It screamed by within just 50 kilometers (31 miles) of Enceladus’ icy surface. It pierced the cloud of water, dust and sea salt spraying from the cracks at its south pole.

"Within 20 seconds the spacecraft is in and out of the plume," says Kempf. And within each second it had measured up to 3,500 of those ice crystals.

Kempf was still combing through these results in May 2016. They showed that the moon's cracks spew some 2.5 kilograms (5.5 pounds) of ice and mineral dust every second.

It confirmed just what he had predicted. "That doesn't happen very often in planetary science," he says. It makes him more confident than ever that he hasn't made some horrible error. It really seems likely that the mineral dust is coming from an ocean deep within Enceladus.

Other evidence also emerged from that flyby. Mission scientists now believe they see traces of hydrogen in the water spraying from Enceladus. Methane, too, has been showing up. This all bolsters the idea that Enceladus offers plenty of food to support life.

Looking for such life inside Enceladus will require sending another spacecraft there. German scientists are designing a crew-less craft to do just that. But it could take nearly 20 years for it to reach Enceladus and land on it.

In the meantime, Cassini’s time studying Enceladus and the other moons of Saturn is drawing to an end. The spacecraft is running out of fuel, so NASA will soon end its mission.

In mid-December of this year, NASA will send Cassini for one last pass by Enceladus. During that event, the probe will measure how much heat is seeping through the ice cracks and into space.

Then, in 2017, mission controllers will use Cassini's remaining fuel to fire its engines. The spacecraft will spiral closer and closer to Saturn. Cassini will see Saturn's innermost rings more closely than ever before. It may even see individual chunks of ice that make up those rings.

Then Cassini will skim over Saturn's misty surface and enter its uppermost clouds. It will slow down as it plows through those thickening clouds. The spacecraft will succumb to Saturn's gravity. Around September 15, 2017, the probe will plummet. And as it does, it eventually will crumple like tinfoil under the weight — many tons per square inch — of Saturn's thick atmosphere.

Kempf knew Cassini’s awesome sightseeing venture would eventually come to an end. But he hopes it will yield one last burst of discoveries in its final hours. "This will be awesome," he predicts. "We will end with fireworks."

Power Words

alien A non-native organism. (in astronomy) Life on or from a distant world.

amphipod An order of water-dwelling crustaceans related to lobsters, shrimp, and krill. Amphipods vaguely resemble shrimp. Most types are less than one centimeter (0.4 inch) long.

Antarctica A continent mostly covered in ice, which sits in the southernmost part of the world.

Apollo 11 The first space flight ever to land humans on the moon. Two astronauts, named Neil Armstrong and Buzz Aldrin, landed there on July 20, 1969. They spent nearly 22 hours on the moon. They collected a handful of rocks and other samples that were returned home for scientists to study.

astronaut Someone trained to travel into space for research and exploration.

Atlantic One of the world’s five oceans, it is second in size only to the Pacific. It separates Europe and Africa to the east from North and South America to the west.

atmosphere The envelope of gases surrounding Earth or another planet.

calcium A chemical element which is common in minerals of the Earth’s crust. It is also found in bone mineral and teeth, and can play a role in the movement of certain substances into and out of cells.

Cassini A space probe sent by NASA to explore the planet Saturn. Cassini was launched from Earth in 1997. It reached Saturn in late 2004. The craft included a variety of instruments meant to study Saturn’s moons, rings, magnetic field and atmosphere.

cell The smallest structural and functional unit of an organism. Typically too small to see with the naked eye, it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size.

chemical A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O. Chemical can also be an adjective that describes properties of materials that are the result of various reactions between different compounds.

chemical reaction A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).

chemistry The field of science that deals with the composition, structure and properties of substances and how they interact with one another. Chemists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) The term is used to refer to the recipe of a compound, the way it’s produced or some of its properties.

cloud A mass of airborne water droplets and ice crystals that travel as a plume, usually high in Earth’s atmosphere. Their movement is driven by winds.

cosmic An adjective that refers to the cosmos — the universe and everything within it.

crater A large, bowl-shaped cavity in the ground or on the surface of a planet or the moon. They are typically caused by an explosion or the impact of a meteorite or other celestial body. Such an impact is sometimes referred to as a cratering event.

crystal (adj. crystalline ) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals.

E-ring A faint ring encircling the planet Saturn. It is Saturn's outermost major ring, and also its widest, measuring 360,000 kilometers (220,000 miles) from edge to edge. The E-ring is composed of tiny crystals of water ice.

electric charge The physical property responsible for electric force; it can be negative or positive.

electric field A region around a charged particle or object within which a force would be exerted on other charged particles or objects.

Enceladus The sixth largest of Saturn’s more than 50 moons. Enceladus is bright white and covered with a thick shell of ice. Deep beneath that ice sits what appears to be a global ocean of salty liquid water. Enceladus is a round sphere, 500 kilometers (310 miles) across. It is a little less than one-third the width of Earth's moon.

engineer A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

eruption The sudden bursting or spraying of hot material from deep inside a planet or moon and out through its surface. In colder parts of the solar system, eruptions often involve liquid water spraying out through cracks in an icy crust. This happens on Enceladus, a moon of Saturn that is covered in ice.

faulted The presence of deep fractures or cracks in the outer crust of a planet or moon. These cracks are caused by movements within the crust.

geyser A vent (opening) in Earth’s surface that intermittently sends up a tall spray of steam or hot water. The sometimes explosive discharge of water and steam is propelled by the geothermal heating of water below ground.

glass A hard, brittle substance made by melting sand (silica) together with lime (a highly alkaline material obtained by heating limestone) and other ingredients, then cooling the mix quickly. Glass usually is transparent and fairly inert (chemically nonreactive).

graduate student Someone working toward an advanced degree by taking classes and performing research. This work is done after the student has already graduated from college (usually with a four-year degree).

gravity The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

hydrogen The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues.

hydrothermal vent Openings at the bottom of the ocean or a lake where hot water emerges from deep inside the earth. The water is rich in minerals and chemicals that can nourish ecosystems of worms, clams, microbes and other organisms.

iron A chemical element which is common in minerals of the Earth’s crust and in its hot core. This metal is also found in cosmic dust, and in many meteorites that fall to Earth from space.

magnesium A metallic element that is number 12 on the periodic table. It burns with a white light and is the eighth most abundant element in the Earth’s crust.

massif (in geology) Part of a mountain or mountain range that is independent of the neighboring rock.

meteor A lump of rock or metal from space that hits the atmosphere of Earth. In space it is known as a meteoroid. When you see it in the sky it is a meteor. And when it hits the ground it is called a meteorite.

methane A hydrocarbon with the chemical formula CH4 (meaning there are four hydrogen atoms bound to one carbon atom). It’s a natural constituent of what’s known as natural gas. It’s also emitted by decomposing plant material in wetlands and is belched out by cows and other ruminant livestock. From a climate perspective, methane is 20 times more potent than carbon dioxide is in trapping heat in Earth’s atmosphere, making it a very important greenhouse gas.

microbe Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

microbiologist A scientist who studies microorganisms such as bacteria, fungi and viruses, how they cause infections, and how they interact with their environment.

mineral The crystal-forming substances, such as quartz, apatite, or various carbonates, that make up rock. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in certain regular three-dimensional patterns

molecule A group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

moon The natural satellite of any planet.

multicellular Having or consisting of many cells. This includes all animals and plants, and many types of fungus.

NASA (National Aeronautics and Space Administration) Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It has also sent research craft to study planets and other celestial objects in our solar system.

orbit The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.

organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.

oxygen A gas that makes up about 21 percent of Earth’s atmosphere. It is a common component in minerals (such as silicate) in Earth’s crust. It is also a component of water. All animals and many microorganisms need oxygen to fuel their metabolism.

PhD (also known as a doctorate) A type of advanced degree offered by universities — typically after five or six years of study — for work that creates new knowledge. People qualify to begin this type of graduate study only after having first completed a college degree (a program that typically takes four years of study).

planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now consists of eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

planetary science The science of other planets besides Earth.

plume (in geology) Fluids — air, water or magma typically — that move, largely intact, in a feather-like shape over long distances.

primordial An adjective that refers to something that goes back to the beginning of time or to the earliest existence of something.

radiation (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) Electromagnetic waves carry radiating energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

red blood cell Colored red by hemoglobin, these cells move oxygen from the lungs to all tissues of the body. Red blood cells are too small to be seen by the naked eye.

reflective Having the quality of reflecting light strongly. Reflective objects can produce a strong bright glare when sunlight bounces off of them. Examples of reflective objects include a mirror, a smooth metal can, a car window, a glass bottle, ice, snow or the watery surface of a lake.

salt A compound made by combining an acid with a base (in a reaction that also creates water). The ocean contains many different salts — collectively called “sea salt.” Common table salt is a made of sodium and chlorine.

Saturn The sixth planet out from the sun in our solar system. One of the four gas giants, this planet takes 10.7 hours to rotate (completing a day) and 29 Earth years to complete one orbit of the sun. It has at least 53 known moons and 9 more candidates awaiting confirmation. But what most distinguishes this planet is the broad and flat plane of seven rings that orbit it.

serpentinization A chemical process that occurs when sea water seeps far, far below the sea floor and encounters iron-rich minerals that have never before touched water. The reaction oxidizes the iron — essentially “rusting” it really quickly. This produces this heat and leaves the water alkaline (with a high pH).

silicate A mineral containing silicon atoms and usually oxygen atoms. The majority of Earth’s crust is made of silicate minerals.

sodium chloride A type of salt containing the chemical elements sodium and chlorine. Sodium chloride is the main component of sea salt. It is used as table salt.

solar system The eight major planets and their moons in orbit around the sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.

star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

story (in measurements) A distance equal to the height of one floor of a building, usually about 3 meters (10 feet).

Texas The second largest state in the United States, located along the southern border with Mexico. It is about 1,270 kilometers (790 miles) long and covers an area of 696,000 square kilometers (268,581 square miles).

trillion A number representing a million million — or 1,000,000,000,000 — of something.

water vapor Water in its gaseous state, capable of being suspended in the air.

weather Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.

wreckfish A type of ocean fish that often lives around rocks, shipwrecks or undersea caves.